Universal joints are used in mechanical applications to transmit torque between components where there can be misalignment of rotating parts. In a drilling operation, a drill bit is mounted to the end of a drill string. The drill string is rotated from the top of the string or by a motor at the bottom of the string, or both, to rotate the drill bit and advance the borehole. Universal joints are included in the drill string to accommodate rotational eccentricity. The eccentric rotation is converted into axial rotation in order for the drill bit to advance the borehole efficiently. Eccentricity can be initiated by a motor in the drive assembly that rotates the drill bit or by steering of the bit to change direction of the borehole.
FIG. 1 is a schematic representation of a drilling operation 2. In conventional drilling operations a drill bit 8 is mounted on the end of a drill string 6 comprising drill pipe and drill collars. The pipe sections of the drill string are threaded together at their ends to create a pipe of sufficient length to reach the bottom of the wellbore 4. The drill string may be several miles long. The bit is rotated in the bore either by a motor proximate to the bit or by rotating the drill string or both simultaneously. A pump circulates drilling fluid through the drill pipe and out of the drill bit, flushing rock cuttings from the bit and transporting them back up the wellbore. Additional tools and components 10 can be included in the drill string such as motors and vibrators.
The components of the drill string including the universal joint are subjected to extreme torque forces, elevated operating temperatures and abrasive drilling fluids, all of which can have an adverse effect on the operational life of drill string components. Constant relative movement of the components of a universal joint during operations, together with abrasive drilling mud, causes abrasion and erosion of mating components. Attempts have been made to effectively seal the universal joint assemblies so as to prolong their operational life. However, the constant relative movement of the components and aggressive downhole environment leads to difficulties in conventional sealing arrangements. Replacement of the joint or its components requires removal of the drill string from the borehole and downtime for the operation, which increases operational expenses substantially.
A universal joint that is less vulnerable to abrasion and erosion with an extended service life would be advantageous.